The 2017 curriculum and academisation has a lot to answer for imo, with Ofsted and COVID as cherry on the cake.
I have worked in SEND for almost 30 years.
A lot of the "mental health" issues affecting young people are artefacts of an absolute awful education policy around exams. These are generally skewed so hard that even children who are academically able feel like failures. The score for a grade 4 "pass" in maths hovers around 26 percent. The score for a grade 6 in science - equivalent to a high grade B - is around 46 percent. Grade 9 in biology, attained by the top few percent, is around 67 percent. There is absolutely no need for exams so hard that even able kids in the top 20 percent are getting less than half marks. Imagine if you actually struggle in the subject how daunting those papers are. English language no longer assesses ability to read, comprehend, spell, and write coherently. It is now an unseen literature paper requiring commentary on authorial intent - incredibly hard for many autistic kids who can read, spell, and write to high level.
The curriculum review made subjects so packed and so difficult that swathes of children have little chance, around 40 percent "fail" by design in many subjects.
Added to intransigent policies around things like uniform, eye contact, etc in some academy chains, that take no account of neurodiversity, and measures like progress 8 that are entirely GCSE focused and don't encourage schools to let children study qualifications at their level (such as entry level or functional skills), and OFSTED until recently entirely uninterested in SEND but focusing on "standards" and it's no wonder our kids are struggling.
If you enabled a self study option GCSE for kids at GCSE level - a bit like the EPQ some do at sixth form - and created a menu of entry level, functional skills and GCSE in every secondary school, you would cut down on "SEND " in mainstream hugely. SEND just means "this environment doesn't match this child's needs". Why can't we make the environment more flexible and responsive in that case?